Semiconductor device and fabrication method for the same

ABSTRACT

A fabrication method for a semiconductor device includes the step of forming a gate insulating film on the side of a trench, the bottom thereof, and the periphery thereof. The step of forming a gate insulating film includes a step of forming a first insulating film on the side of the trench and a step of forming a second insulating film on the bottom and periphery of the trench using a high-density plasma chemical vapor deposition method. The thickness of the portions of the gate insulating film formed on the bottom and periphery of the trench is made larger than that of the portion of the gate insulating film formed on the side of the trench.

TECHNICAL FIELD

The present disclosure relates to a semiconductor device and afabrication method for the same, and more particularly to asemiconductor device having a trench gate structure and a fabricationmethod for the same.

BACKGROUND ART

In recent years, semiconductor devices having a trench gate structurehave attracted attention. While a channel is formed on the surface of asemiconductor layer in semiconductor devices having a planar gatestructure, a channel region is formed on the side of a trench providedin a semiconductor layer in the semiconductor devices having the trenchgate structure. The trench gate structure semiconductor devices aretherefore more expected to achieve miniaturization and reduction in ONresistance than the planar gate structure semiconductor devices. Forthis reason, in the field of power devices, in particular, developmentof trench gate structure semiconductor devices is underway.

The trench gate structure semiconductor devices, which are free from thelimitation on miniaturization caused by the junction FET (JFET) effect,have an advantage that a fine trench can be formed to permit reductionin ON resistance and switching loss. However, a problem arises that theaspect ratio of a fine trench is large, making it difficult to embed agate electrode in such a trench. Also, since the cross-sectional area ofthe gate electrode decreases, the gate resistance will increase. Toprevent or reduce the increase in gate resistance, it is being examinedto form a gate electrode in a T shape to extend the gate electrode overthe periphery of the trench (see Patent Document 1, for example).

Also, in the trench gate structure semiconductor devices, it isimportant to form a suitable gate insulating film inside the trench. Anelectric field is concentrated on the bottom of the trench, where thefield strength is higher than in any other portion. Therefore, there isthe possibility that a breakdown due to a dielectric breakdown of thegate insulating film may occur at the bottom of the trench.

If the gate insulating film is thickened as a whole to increase thedielectric breakdown field, the threshold voltage at switching willincrease. Therefore, a method of forming a thicker gate insulating filmon the bottom of the trench using a difference in plane directionbetween the side and bottom surfaces of the trench is being examined(see Patent Document 2, for example). Also being examined is a methodwhere a mask is formed on the side of the trench during formation of thegate insulating film to form a thick gate insulating film on the bottomof the trench while preventing or reducing formation of an oxide film onthe side of the trench (see Patent Document 3, for example).

CITATION LIST Patent Document

-   PATENT DOCUMENT 1: Japanese Patent Publication No. 2007-281512-   PATENT DOCUMENT 2: Japanese Patent Publication No. H07-326755-   PATENT DOCUMENT 3: Japanese Patent Publication No. 2007-242943

SUMMARY OF THE INVENTION Technical Problem

However, the conventional trench gate structure semiconductor devicesdescribed above have the following problems. First, in the case ofcontrolling the thickness of the gate insulating film using the planedirection of the substrate, a substrate having a special plane directionis necessary, and this increases the fabrication cost. Also, it is notpossible to set the film thickness on the side of the trench and that onthe bottom thereof to arbitrary values independently.

In the case of forming a mask on the side of the trench to form a thickgate insulating film only on the bottom of the trench, steps of formingand removing the mask are necessary. This complicates the fabricationprocess, and increases the fabrication cost and the cycle time.

In the case of forming a T-shaped gate electrode, it is necessary toform the gate insulating film also on the portion of the semiconductorlayer surrounding the trench. If the gate insulating film formed on theperiphery of the trench is thin, the gate-source capacitance willincrease, causing a delay. In formation of a T-shaped gate electrode,therefore, it is necessary to control not only the thickness of theportions of the gate insulating film on the side and bottom of thetrench, but also the thickness of the portion thereof on the peripheryof the trench.

Moreover, if a thick gate insulating film is formed on the periphery ofthe trench, the trench will become virtually deep, increasing the aspectratio. This raises a problem of making it difficult to embed the gateelectrode in the trench.

The problems described above occur commonly in silicon semiconductordevices and in semiconductor devices using a wide band-gap semiconductorsuch as silicon carbide (SiC). The relative permittivity of SiC (9.7 for4H—SiC) is smaller than that of Si (11.9), and the difference thereoffrom that of SiO₂ (3.8) is small. Therefore, in a semiconductor deviceusing SiC, a larger electric field is applied to the gate insulatingfilm, causing a larger problem.

According to an embodiment disclosed in this specification, asemiconductor device is provided where control of the thickness of thegate insulating film inside the trench and on the periphery of thetrench is easy and also embedding of the gate electrode into the trenchis facilitated.

Solution to the Problem

One form of the fabrication method for a semiconductor device disclosedin this specification includes the steps of preparing a substrate havinga semiconductor layer provided on a principal surface; forming a trenchin the semiconductor layer; forming a gate insulating film on a side ofthe trench, a bottom of the trench, and a periphery of the trench; andforming a conductive film on the gate insulating film to fill the trenchand extend on the periphery of the trench. The step of forming a gateinsulating film includes a step of forming a first insulating film onthe side of the trench and a step of forming a second insulating film onthe bottom of the trench and the periphery of the trench using ahigh-density plasma chemical vapor deposition method, the thickness ofportions of the gate insulating film formed on the bottom of the trenchand the periphery of the trench being made larger than that of a portionof the gate insulating film formed on the side of the trench. In thestep of forming a conductive film, the conductive film is formed to bein contact with a portion of the first insulating film formed on theside of the trench.

One form of the semiconductor device disclosed in this specificationincludes: a substrate; a semiconductor layer provided on a principalsurface of the substrate; a trench provided in the semiconductor layer;a gate insulating film provided on a side of the trench, a bottom of thetrench, and a periphery of the trench; and a conductive film provided onthe gate insulating film to fill the trench and extend on the peripheryof the trench, wherein the gate insulating film has a first insulatingfilm provided on the side of the trench and a second insulating filmprovided on the bottom of the trench and the periphery of the trench,the thickness of portions of the gate insulating film provided on thebottom of the trench and the periphery of the trench is larger than thatof a portion of the gate insulating film provided on the side of thetrench, a portion of the second insulating film provided on theperiphery of the trench has an inclined portion that becomes graduallythicker from the trench-side end, and the inclination angle of theinclined portion with respect to the principal surface of the substrateis 45±5 degrees, and the conductive film is in contact with a portion ofthe first insulating film formed on the side of the trench.

Advantages of the Invention

According to an embodiment disclosed in this specification, it ispossible to implement a semiconductor device where control of thethickness of the gate insulating film inside the trench and on theperiphery of the trench is easy and also embedding of the gate electrodeinto the trench is facilitated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the thickness of aninsulating film formed on the bottom of a trench and the field strengthexerted on the insulating film at the bottom of the trench.

FIG. 2 is a plan view showing a semiconductor device of an embodiment.

FIG. 3 is a cross-sectional view showing the semiconductor device of theembodiment.

FIG. 4 is a cross-sectional view showing a fabrication process for thesemiconductor device of the embodiment.

FIG. 5 is a cross-sectional view showing the fabrication process for thesemiconductor device of the embodiment.

FIG. 6 is a cross-sectional view showing the fabrication process for thesemiconductor device of the embodiment.

FIG. 7 is a cross-sectional view showing the fabrication process for thesemiconductor device of the embodiment.

FIG. 8 is a cross-sectional view showing the fabrication process for thesemiconductor device of the embodiment.

FIG. 9 is a cross-sectional view showing the fabrication process for thesemiconductor device of the embodiment.

FIG. 10 is a cross-sectional view showing the fabrication process forthe semiconductor device of the embodiment.

FIGS. 11( a)-11(c) are cross-sectional views showing a first alterationof the fabrication process for the semiconductor device of theembodiment.

FIG. 12 is an enlarged cross-sectional view showing a second alterationof the semiconductor device of the embodiment.

FIG. 13 is an enlarged cross-sectional view showing a third alterationof the semiconductor device of the embodiment.

FIG. 14 is a cross-sectional view showing a fourth alteration of thefabrication process for the semiconductor device of the embodiment.

FIG. 15 is a cross-sectional view showing the fourth alteration of thefabrication process for the semiconductor device of the embodiment.

FIG. 16 is a cross-sectional view showing the fourth alteration of thefabrication process for the semiconductor device of the embodiment.

FIG. 17 is a cross-sectional view showing a fifth alteration of thesemiconductor device of the embodiment.

FIG. 18 is a cross-sectional view showing a sixth alteration of thesemiconductor device of the embodiment.

FIG. 19 is a cross-sectional view showing a seventh alteration of thesemiconductor device of the embodiment.

FIG. 20 is a cross-sectional view showing an eighth alteration of thesemiconductor device of the embodiment.

DESCRIPTION OF EMBODIMENTS

In the fabrication method for a semiconductor device disclosed in thisspecification, a gate insulating film is formed by a combination of ahigh-density plasma chemical vapor deposition (HDP-CVD) method and athermal oxidation method, for example.

According to the above fabrication method, it is possible to freely setthe thickness of a gate insulating film on the bottom and periphery of atrench and the thickness of a gate insulating film on the side of thetrench independently without complicating the process. It is alsopossible to easily implement a semiconductor device that can prevent orreduce occurrence of dielectric breakdown of the gate insulating film onthe bottom of a trench with little influence exerted on properties suchas the threshold voltage. Moreover, it is possible to implement asemiconductor device that can achieve both reduction in gate resistanceand reduction in gate capacitance and is excellent in high frequencyoperation.

First, the required thickness of the gate insulating film will bedescribed. FIG. 1 shows the results, obtained by simulation, of thefield strength exerted on the trench bottom in a trench gate structuresemiconductor device using 4H—SiC. The drain voltage was set to 1200 Vand the junction breakdown voltage between a drift region and a bodyregion to 1200 V or more. The field strength exerted on the trenchbottom when the thickness of the gate insulating film on the trenchbottom was varied while the thickness thereof in a channel region on thetrench side was fixed to 70 nm was obtained.

As shown in FIG. 1, when the thickness of the gate insulating film onthe trench bottom is 70 nm, which is the same as that on the trenchside, the field strength exceeds 9 MV/cm. Even when the film thicknesson the trench bottom is 140 nm, which is twice as large as that on thetrench side, a field strength of 6 MV/cm is exerted on the trenchbottom.

The dielectric breakdown field strength of a normal thermally-oxidizedfilm is 10 mV/cm or more. However, to ensure the reliability inlong-term use, it is preferable to set the field strength allowable fora semiconductor device to on the order of 3 MV/cm to 4 MV/cm, which issufficiently smaller than the dielectric breakdown field strength.

As shown in FIG. 1, by setting the thickness of the gate insulating filmon the trench bottom to 350 nm or more, which is five times or more aslarge as the thickness of the gate insulating film on the trench side,the field strength exerted on the trench bottom becomes 4 MV/cm or less.

In 4H—SiC, the (000-1) C plane is a plane large in oxidation speed, butthe oxidation speed thereof is only about twice as high as that of the(11-20) plane. Therefore, it is difficult to secure, as the thickness ofthe gate insulating film on the trench bottom, a thickness five times ormore as large as that on the trench side by using the difference inoxidation speed between the plane directions. Moreover, since thecombination of the plane direction of the bottom surface of the trenchand the plane direction of the side surface thereof is restrictive, itis not possible to control the film thickness on the side and the filmthickness on the bottom independently. Furthermore, on the C plane,low-density epitaxial growth is difficult. For these reasons, it isdifficult and unpractical to reduce the electric field exerted on thetrench bottom to a predetermined value or less, while securing theproperties of the transistor, using the difference in oxidation speedbetween the plane directions.

In the case of thickening the gate insulating film on the trench bottomby forming the gate insulating film in a plurality of separate steps, itis possible to control the film thickness on the trench side and thefilm thickness on the trench bottom independently. However, this raisesa large problem of increasing the fabrication steps. For example, thefollowing steps will be necessary. First, after formation of athermally-oxidized film inside the trench, a polysilicon film is formedto cover the thermally-oxidized film. Thereafter, a nitride filmcovering the polysilicon film is formed and then selectively removed, toform a mask covering the side of the trench and exposing the bottomthereof. After thermal oxidation of the exposed portion of thepolysilicon film, the mask is removed, and further the unoxidizedpolysilicon film is removed.

The oxidized film obtained by oxidizing the polysilicon film is low indielectric breakdown field strength compared with an oxidized filmobtained by oxidizing single-crystalline silicon. In this case,therefore, the gate insulating film on the trench bottom must be madethicker than that in the case of directly forming a thermally-oxidizedfilm on the trench bottom. It is however difficult to completely oxidizea thick polysilicon film down to its inside, and thus it is not easy tolargely increase the thickness of the gate insulating film on the trenchbottom compared with that on the trench side.

Moreover, in either of the above methods, a thick insulating film isformed on the trench bottom by thermal oxidation. The present inventorshave found that, when a thermally-oxidized film having a thickness twiceor more as large as that on the trench side is formed on the trenchbottom, a defect tends to be introduced in the semiconductor layer. Inthe process of forming a thermally-oxidized film, the volume of thesurface portion of the semiconductor layer increases by oxidation.Therefore, when a thick thermally-oxidized film is formed on the trenchbottom, a large stress will be exerted on the corners of the trenchbottom, increasing the possibility of disturbing the crystallinity atthe corners. This tends to cause a defect in the semiconductor layer,resulting in the possibilities of reducing the breakdown voltage of thesemiconductor device and increasing the leak current.

The present inventors have focused attention on the fact that, by usingthe HDP-CVD method, an SiO₂ film can be selectively deposited on thebottom of a trench while being little deposited on the side thereof. Byusing the HDP-CVD method, it is possible to control the film thicknesson the side of the trench and the film thickness on the bottom thereofindependently. Also, with no mask needed, the number of process stepswill not largely increase. Moreover, since a dense SiO₂ film can beobtained using the HDP-CVD method, a large dielectric breakdown fieldstrength can be obtained even if the thickness of this film is smallcompared with that of an SiO₂ film obtained by oxidizing polysilicon.

In the formation of an SiO₂ film using the HDP-CVD method, filmdeposition and etching occur simultaneously, to etch an edge portion.Therefore, an SiO₂ film formed on the periphery of the trench has aninclined portion that becomes gradually thicker from the trench side.Accordingly, even though a comparatively thick gate insulating film isformed on the periphery of the trench, an increase in the virtual aspectratio of the trench can be prevented or reduced, and also an advantageof facilitating embedding of a gate electrode can be obtained.

The semiconductor device of the present disclosure will be describedhereinafter by way of example. The semiconductor device of thisembodiment is a SiC-metal-insulator-semiconductor field effecttransistor (MISFET) having the trench gate structure, and has aplurality of unit cells 11. FIG. 2 shows a planar configuration of theunit cells 11, and FIG. 3 shows a cross-sectional configuration takenalong line in FIG. 2. Note that illustration of source electrodes isomitted in FIG. 2. Each unit cell 11 has a semiconductor layer 102provided on the surface (principal surface) of a substrate 101 includingSiC. The semiconductor layer 102 has an n-type drift region 121, ap-type body region provided on the drift region 121, and an n-typesource region 124 provided in a top portion of the body region 123. Thesource region 124 is surrounded by the body region 123.

The semiconductor layer 102 has a trench (concave) 102 a extendingthrough the source region 124 and the body region 123 to reach the driftregion 121. A gate insulating film 103 is formed inside the trench 102 aand on a portion of the source region 124 surrounding the trench 102 a.The gate insulating film 103 includes a first insulating film 131provided on the side of the trench 102 a and a second insulating film132 provided on the bottom of the trench 102 a and the periphery of thetrench 102 a. The second insulating film 132 includes an inner-trenchportion 132A provided on the bottom of the trench 102 a and atrench-periphery portion 132B provided on a portion of the semiconductorlayer 102 surrounding the trench 102 a.

A gate electrode 105 made of a conductive film is embedded in the trench102 a.

The gate electrode 105 is formed in a T shape in cross section extendingover the periphery of the trench 102 a. The bottom of the portion of thegate electrode 105 inside the trench 102 a is in contact with theinner-trench portion 132A of the second insulating film 132, and theside thereof is in contact with the first insulating film 131. Theportion of the gate electrode 105 located on the periphery of the trench102 a is in contact with the trench-periphery portion 132B of the secondinsulating film 132. Thus, the gate electrode 105 is insulated from thesemiconductor layer 102 by the gate insulating film 103.

A source electrode (source-body electrode) 106 electrically connected tothe source region 124 and the body region 123 is provided on thesemiconductor layer 102. The source electrode 106 is formed to surroundthe trench 102 a. A drain electrode 107 is provided on the back surfaceof the substrate 101. An interlayer insulating film covering the gateelectrode 105 and the source electrode 106 is provided on thesemiconductor layer 102 as required, and a gate interconnect and asource interconnect are provided in the interlayer insulating film. Thegate interconnect and the source interconnect are respectively connectedto the gate electrode 105 and the source electrode 106 via respectivecontact plugs.

The semiconductor device of this embodiment may be fabricated in thefollowing manner. First, as shown in FIG. 4, SiC is epitaxially grown onthe principal surface of the substrate 101, to form the n-type driftregion 121 and the p-type body region 123 sequentially. Subsequently,the n-type source region 124 is formed in the body region 123. As thesubstrate 101, a low-resistance n-type SiC substrate containing nitrogenin a concentration of about 3×10¹⁸ cm⁻³ can be used. The drift region121 may be doped with nitrogen in a concentration of about 8×10¹⁵ cm⁻³.The thickness of the drift region 121 may be about 12 μm. The impurityconcentration and thickness of the drift region 121 are not limited tothe above examples, but may be determined depending on the breakdownvoltage required.

The body region 123 may be doped with aluminum in a concentration ofabout 2×10¹⁸ cm⁻³. The thickness of the body region 123 may be about 1μm.

The source region 124 may be formed by ion implantation, etc. Animplantation mask made of SiO₂, etc. may be formed so as to expose aportion of the body region 123 that is to be the source region 124, andthereafter n-type impurity ions (e.g. nitrogen ions) may be implanted inthe body region 123. The ion implantation may be performed under theconditions of an accelerating energy of 100 keV and a dose of 5×10¹⁵cm⁻². After removal of the implantation mask, annealing may be performedat a temperature of about 1700° C. under an inert gas atmosphere forabout 30 minutes. By this annealing, the implanted impurity ions areactivated to provide the source region 124.

While an example of forming the semiconductor layer 102 by epitaxialgrowth has been described, the entirety or part of the semiconductorlayer 102 may be formed by performing ion implantation, etc. for a SiCsubstrate. For example, a p-type impurity may be implanted in the n-typeSiC substrate to use an upper portion of the SiC substrate as the bodyregion 123. Alternatively, after epitaxial growth of an n-typesemiconductor layer on a SiC substrate, p-type impurity ions may beimplanted in a surface region of the grown n-type semiconductor layer,to form the body region 123. In these cases, the region having no p-typeimpurity implanted serves as the drift region 121.

Thereafter, as shown in FIG. 5, the trench 102 a is formed in thesemiconductor layer 102. For example, part of the source region 124, thebody region 123 and the drift region 121 may be removed by reactive ionetching (RIE) using a mask made of an SiO₂ film that exposes a centerportion of the source region 124, to form the trench 102 a. The trench102 a is formed so as not to extend through the drift region 121 and tohave its bottom located below the interface between the drift region 121and the body region 123. For example, the trench 102 a may have a depthof about 1.5 μm and a width of about 1 μm. While an example of thetrench 102 a the side of which is vertical to the principal surface ofthe substrate 101 is shown in FIG. 5, the side of the trench 102 a maynot be necessarily vertical to the principal surface of the substrate101.

Thereafter, as shown in FIG. 6, the second insulating film 132 isdeposited by the HDP-CVD method. By using the HDP-CVD method, controlcan be made so as to form the second insulating film 132 on the bottomof the trench 102 a and the periphery thereof but not to form on theside of the trench 102 a. The thickness of the inner-trench portion 132Aformed on the bottom of the trench 102 a and the trench-peripheryportion 132B formed on the periphery of the trench 102 a may be set toabout 150 nm to about 500 nm. In this embodiment, the thickness of theseportions is set to about 350 nm as an example. The thickness of thesecond insulating film 132 may be adjusted by the deposition time.

After the formation of the second insulating film 132, in order toimprove the insulation property of the second insulating film 132, it ispreferable to perform heat treatment at a temperature of about 900° C.under an inert gas atmosphere or an oxidation atmosphere. Such heattreatment may however be omitted.

In this embodiment, the second insulating film 132 is formed by theHDP-CVD method. The HDP-CVD method is a CVD method using high-densityplasma, where the plasma density is higher than in normal plasma CVD,and has advantages such as that a good-quality film can be formed evenat a low temperature. The HDP-CVD method also has a feature that, sincesputter-etching and deposition are performed simultaneously, a film islittle formed on a portion of the underlying layer inclined at a degreehigher than about 45 degrees, and the end of a film formed gives aninclined face having an inclination angle of about 45 degrees.Therefore, as shown in FIG. 6, the second insulating film 132 is littledeposited on the side of the trench 102 a. Thus, the second insulatingfilm 132 can be formed only on the bottom of the trench 102 a and theperiphery thereof without the necessity of forming a mask on the side ofthe trench 102 a.

By the HDP-CVD method, even when a thick SiO₂ film is formed, a largestress is less likely to occur on the substrate compared with the caseof forming an SiO₂ film by the thermal oxidation method. This methodtherefore provides another advantage that, even when a thick SiO₂ filmis formed on the bottom of the trench 102 a, it is possible to preventor reduce introduction of a defect in the semiconductor layer 102.

The formation of the second insulating film 132 by the HDP-CVD methodcan be performed using a known apparatus under known conditions. Forexample, when using an HDP-CVD apparatus of a parallel plate type havingtop and side plasma generation coils, an SiO₂ film may be depositedunder the following conditions. The electric power supplied to the topcoil may be about 1300 W and that to the side coil about 3100 W, toapply a bias of about 3300 W to the substrate. Into the chamber having apressure of about 6 mTorr (about 0.8 Pa), fed are argon (Ar) at about125 sccm (sccm means ml/min at 0° C. and 1 atm.; about 0.21 Pa m³/s),oxygen (O₂) at about 145 sccm (about 0.25 Pa m³/s), and silane (SiH₄) atabout 80 sccm (about 0.14 Pa m³/s). Argon, oxygen, and silane are fedfrom top and from side. The feed amounts of argon, oxygen, and silanemay be adjusted according to the growth rate required.

Thereafter, as shown in FIG. 7, the first insulating film 131 is formedon the side of the trench 102 a. The first insulating film 131 may bemade thinner than the second insulating film 132. Specifically, thethickness may be about 30 nm to about 100 nm. For example, by performingheat treatment at a temperature of about 1200° C. under a dry oxidationatmosphere for about three hours, the first insulating film 131 having athickness of about 70 nm can be formed. During this heat treatment,also, an effect of densifying the second insulating film 132 isobtained.

In the thermal oxidation, the first insulating film 131 grows toward thesemiconductor layer 102 and toward the trench 102 a approximatelyequally. Therefore, as shown in FIG. 7, the end of the trench-peripheryportion 132B of the second insulating film 132 on the trench 102 a sideis now located at a position somewhat receding from the top edge of thetrench 102 a after formation of the first insulating film 131.Theoretically, the end of the trench-periphery portion 132B is at theposition of the top edge of the trench 102 a before formation of thefirst insulating film 131. When the first insulating film 131 is growntoward the semiconductor layer 102 and toward the trench 102 aapproximately equally, the spacing t1 between the end of thetrench-periphery portion 132B and the top edge of the trench 102 a afterthe formation of the first insulating film 131 will be approximatelyequal to a half of the thickness t2 of the first insulating film 131.

Thereafter, as shown in FIG. 8, a conductive film 105A is formed overthe entire surface of the semiconductor layer 102 including the insideof the trench 102 a. The conductive film 105A may be a polysilicon filmhaving a thickness of about 600 nm doped with phosphorus in aconcentration of 1×10²⁰ cm⁻³ or more, for example. Such a polysiliconfilm may be formed by an LP-CVD method, for example. Note however thatthe conductive film 105A may be a conductive film other than thepolysilicon film.

Then, as shown in FIG. 9, the conductive film 105A is etched(dry-etched) using a resist layer 141 that covers a portion above thetrench 102 a and exposes the other portion as a mask, to obtain the gateelectrode 105 having a T-shaped cross section. It is preferable that thesize of the portion of the gate electrode 105 hanging over the peripheryof the trench 102 s be 500 nm or more considering a misalignment, etc.in the lithography technique. Note however that the gate electrode 105does not necessarily have a T-shaped cross section as far as the gateresistance required can be secured. Subsequently, the portion of thesecond insulating film 132 that is not covered by the gate electrode 105is removed, to expose the source region 124 and the body region 123.

Thereafter, as shown in FIG. 10, the source electrode 106 is formed soas to be in contact with the body region 123 and the source region 124.Specifically, the source electrode 106 may be formed in the followingmanner. First, an insulating film that is to be an interlayer insulatingfilm is formed to cover the semiconductor layer 102 and the gateelectrode 105. Subsequently, an opening that exposes part of the bodyregion 123 and the source region 124 is formed through the insulatingfilm, and a conductive film is formed inside the opening. The conductivefilm may be a nickel (Ni) film, etc., and may be subjected to heattreatment after the formation. In this way, the source electrode that isin ohmic contact with the source region 124 and the body region 123 isobtained. Also, the drain electrode 107 is formed on the back surface(surface opposite to the principal surface) of the substrate 101.Although not shown, interconnects, plugs for connecting the electrodeswith the interconnects, etc. may be formed as required.

In this embodiment, the gate insulating film 103 is formed by theHDP-CVD method and the thermal oxidation method in combination.Therefore, the thickness of the gate insulating film 103 on the side ofthe trench 102 a and the thickness of the gate insulating film 103 onthe bottom and periphery of the trench 102 a can be easily setindependently without the necessity of forming a mask, etc. Thethicknesses of the gate insulating film 103 on the side of the trench102 a and on the bottom thereof may be set appropriately depending onthe threshold voltage, the breakdown voltage, etc. required. In order toreduce the field strength exerted on the gate insulating film 103 at thebottom of the trench 102 a, the film thickness on the bottom of thetrench 102 a may be made larger than that on the side of the trench 102a. It is more preferable to make the film thickness on the bottom of thetrench 102 a three times or more as large as that on the side of thetrench 102 a. Further, in order to reduce the field strength exerted onthe bottom of the trench 102 a, the film thickness may be made four tofive times as large as that on the side of the trench 102 a.

For example, the thickness of the gate insulating film 103 on the sideof the trench 102 a may be set to about 70 nm, and the thickness of thegate insulating film 103 on the bottom of the trench 102 a to about 350nm to about 400 nm. By setting the thickness of the gate insulating film103 on the bottom of the trench 102 a to about 350 nm to about 400 nm,the field strength exerted on the bottom of the trench 102 a can bereduced to as small as about 4 MV/cm.

In SiC, the oxidation speed of the (0001) Si plane is very low.Therefore, if the (0001) Si plane easy in crystal growth is used as theprincipal surface, and the thickness of the gate insulating film 103 onthe side of the trench 102 a is set to about 70 nm, the thickness of anSiO₂ film formed on the top surface of the semiconductor layer 102 bythe thermal oxidation method will be about 10 nm. Therefore, the gateelectrode 105 having a T-shaped cross section will become close to thesource region 124, increasing the source-gate capacitance.

By contrast, in this embodiment, the thickness of the portion of thegate insulating film 103 formed on the periphery of the trench 102 a isapproximately equal to that on the bottom of the trench 102 a. Forexample, it is easy to set the thickness of the portion of the gateinsulating film 103 formed on the periphery of the trench 102 a to about400 nm. This thickness is about 40 times as large as that of a filmformed on the (0001) Si plane by the thermal oxidation method, and thusthe gate-source capacitance can be reduced to about one-fortieth of thatin the case of the thermal oxidation method. Thus, it is preferable thatthe thickness of the portion of the gate insulating film 103 formed onthe periphery of the trench 102 a be larger than that of the portionthereof formed on the side of the trench 102 a because the gate-sourcecapacitance can be reduced.

It is also preferable that the thickness of the portion of the gateinsulating film 103 formed on the periphery of the trench 102 a belarger than that of the portion thereof formed on the bottom of thetrench 102 a because the trench 102 a can be made shallow. When thetrench 102 a can be made shallow, the time required for formation of thetrench 102 a can be shortened. Moreover, if the trench 102 a is deep,increasing the aspect ratio that is the ratio of the depth to the width,the etching rate will become high near the bottom of the trench 102 a,easily causing a shape degradation called a sub-trench. By reducing thedepth of the trench 102 a, therefore, occurrence of a sub-trench can beprevented or reduced.

In the HDP-CVD method, deposition is more likely to occur on theperiphery of the trench than on the bottom of the trench. In particular,this effect is larger when the aspect ratio of the trench is larger.Therefore, in general, the thickness of the portion (132B) of the gateinsulating film 103 formed on the periphery of the trench 102 a islarger than that of the portion (132A) formed on the bottom of thetrench 102 a. Also, the thickness of the portion (132B) of the gateinsulating film 103 formed on the periphery of the trench 102 a islarger than that of the portion (131) formed on the side of the trench102 a. Note that the portion (132B) of the gate insulating film 103formed on the periphery of the trench 102 a is inclined in theneighborhood of the trench 102 a: the film thickness becomes graduallylarger as the position is farther from the trench 102 a. It shouldtherefore be noted that the thickness of the portion of the gateinsulating film 103 formed on the periphery of the trench 102 a refersto the maximum thickness of the portion formed on the periphery of thetrench 102 a.

Moreover, by forming the second insulating film 132 by the HDP-CVDmethod, the end face of the trench-periphery portion 132B formed on the,periphery of the trench 102 a can be made inclined with respect to theprincipal surface of the substrate 101 as shown in FIG. 6. Theinclination angle θ1 of the inclined face is about 45 degrees in aportion of at least 80% of the inclined face excluding the top andbottom ends thereof although there are variations of about ±5 degrees.By inclining the end face of the trench-periphery portion 132B, anincrease in the virtual aspect ratio of the trench 102 a after theformation of the gate insulating film 103 can be prevented or reduced.Accordingly, embedding of the conductive film 105A can be facilitated.In this way, it is possible to increase the cross-sectional area of thegate electrode 105 while preventing or reducing an increase insource-gate capacitance, and thus prevent or reduce an increase in gateresistance.

It is preferable that the interface between the second insulating film132 and the gate electrode 105 at the bottom of the trench 102 a belocated below the interface between the drift region 121 and the bodyregion 123. Having this configuration, it is ensured that the secondinsulating film 132 is not formed on the portion of the body region 123exposed to the side of the trench 102 a, which is to be the channelregion. Therefore, there is little possibility that the thick secondinsulating film may have an influence on the threshold voltage, etc.

During the deposition of the second insulating film 132, the side of thetrench 102 a is exposed to the material gas for HDP-CVD. This may causea change in the state of the side of the trench 102 a. Also, dependingon the conditions of the HDP-CVD method, part of the second insulatingfilm 132 may be deposited on the side of the trench 102 a. In such acase, the second insulating film 132 formed on the side of the trench102 a will be sufficiently thin compared with the inner-trench portion132A and the trench-periphery portion 132B of the second insulating film132. Therefore, after the deposition of the second insulating film 132on the semiconductor 102 as shown in FIG. 11( a), or after densifying ofthe second insulating film 132, the second insulating film 132 may bewet-etched. FIGS. 11( a) to 11(c) are cross-sectional views showing afirst alteration of the fabrication process for a semiconductor deviceof the embodiment. By wet-etching the second insulating film 132, aportion of the side of the trench 102 a denatured by the material gasfor HDP-CVD or an SiO₂ film thinly formed on the side of the trench 102a by HDP-CVD can be removed. Thirty percent or less of the thickness ofthe second insulating film 132 will be enough as the etching amount ofthe second insulating film 132.

By the wet etching, as shown in FIG. 11( b), the end of thetrench-periphery portion 132B of the second insulating film 132 recedesfrom the edge of the trench 102 a by t3. Therefore, once the firstinsulating film 131 is formed, as shown in FIG. 11( c), the spacing t1between the end of the trench-periphery portion 132B and the top edge ofthe trench 102 a, i.e., the portion of the first insulating film 131corresponding to the trench top edge will be larger than a half of thethickness t2 of the top end portion of the first insulating film 131.

Even if the receding amount of the second insulating film 132 is large,the first insulating film 131 is to be formed after exposure of thesurface of the semiconductor layer 102. Therefore, exposure of thesemiconductor layer 102 is avoided. With the receding amount being 30%or less of the thickness of the second insulating film 132, the area ofthe receding region is small, little causing an increase in source-gatecapacitance.

With the end of the trench-periphery portion 132B having receded by thewet etching, the virtual aspect ratio of the trench 102 a at the time ofembedding of the conductive film 105A becomes small, providing anadvantage of further facilitating embedding of the conductive film 105A.

FIG. 5 shows the case that the top end portion of the trench 102 a isapproximately square. However, as shown in FIG. 12, the top end portionof the trench 102 a may be rounded to have a curved surface. FIG. 12 isan enlarged cross-sectional view showing a second alteration of thesemiconductor device of the embodiment. By rounding the top end portionof the trench 102 a, the field concentration in the top end portion ofthe trench 102 a can be lessened.

The top end portion of the trench 102 a can be rounded by performing theetching for formation of the trench 102 a starting the etching underconditions with which the deposition amount will increase and making anarrangement to gradually decrease the deposition amount. Otherwise, thetop end portion can be rounded by forming the trench 102 a having asquare edge under normal etching conditions and then performingannealing under a hydrogen atmosphere to round the top end portion. Notonly the top end portion of the trench 102 a, but also the bottom endportion thereof may be curved, whereby the field concentration in thebottom end portion of the trench 102 a can be lessened.

In formation of the second insulating film 132 by the HDP-CVD method,the trench-periphery portion 132B is to be formed on a portion of thetop end portion of the trench 102 a of which the inclination is about 45degrees or less. Therefore, when an upper portion of the trench 102 a iscurved, the inclination θ2 of the top end portion of the trench 102 a incontact with the end of the trench-periphery portion 132B correspondswith the inclination θ1 of the inclined portion of the trench-peripheryportion 132B before formation of the first insulating film 131.

When the side of the trench 102 a is wet-etched after formation of thesecond insulating film 132, the end of the trench-periphery portion 132Brecedes from the edge of the trench 102 a. Therefore, as shown in FIG.13, the inclination θ3 of the top end portion of the trench 102 a incontact with the end of the trench-periphery portion 132B is smallerthan the inclination θ1 of the inclined portion of the trench-peripheryportion 132B before formation of the first insulating film 131. FIG. 13is an enlarged cross-sectional view of a third alteration of thesemiconductor device of the embodiment. In this case, the virtual aspectratio of the trench 102 a at the time of embedding of the conductivefilm 105A becomes small, and this further facilitates embedding of theconductive film 105A.

In this embodiment, described was the example of forming the firstinsulating film 131 after formation of the second insulating film 132.The formation of the first insulating film 131 after formation of thesecond insulating film 132 provides an advantage that the firstinsulating film 131 is free from degradation in film quality that mayotherwise occur if the first insulating film 131 is exposed tohigh-density plasma. This also provides an advantage that the thicknessof the first insulating film 131 will not be reduced by high-densityplasma. However, the first insulating film 131 may be formed firstbefore formation of the second insulating film 132. FIGS. 14 to 16 arecross-sectional views showing a fourth alteration of the fabricationprocess for a semiconductor device of the embodiment. An SiO₂ filmformed by the HDP-CVD method is more likely to cause generation of fixedcharge, etc. at the interface with the semiconductor layer than an SiO₂film formed by the thermal oxidation method. By forming the firstinsulating film 131 by the thermal oxidation method prior to formationof the second insulating film 132, it is possible to obtain an advantagethat generation of fixed charge at the interface between the gateinsulating film 103 and the semiconductor layer 102 can be prevented orreduced.

In this alteration, as shown in FIG. 14, after formation of the trench102 a, thermal oxidation is performed before formation of the secondinsulating film 132, to form the first insulating film 131 on theexposed portion of the semiconductor layer 102. Thereafter, as shown inFIG. 15, the second insulating film 132 is formed on the bottom of thetrench 102 a and the periphery thereof by the HDP-CVD method. Then, asin the case of forming the second insulating film 132 before the firstinsulating film 131, formation of the gate electrode 105, formation ofthe source electrode 106, formation of the drain electrode 107, etc. maybe performed.

In the case of forming the first insulating film 131 before the secondinsulating film 132, as shown in FIG. 16, the position of the end of thetrench-periphery portion 132B of the second insulating film 132corresponds with the top edge of the trench 102 a after formation of thefirst insulating film 131. In other words, the position of the end ofthe trench-periphery portion 132B and the position of the portion of thefirst insulating film 131 corresponding to the top edge of the trench102 a correspond with each other. The formation of the first insulatingfilm 131 before the second insulating film 132 can also be applied evenwhen the top end portion of the trench 102 a is rounded. FIG. 17 is across-sectional view showing a fifth alteration of the semiconductordevice of the embodiment. In this case, as shown in FIG. 17, theinclination θ4 of a portion of the trench 102 a having the firstinsulating film 131 formed thereon that is in contact with the end ofthe trench-periphery portion 132B corresponds with the inclination θ1 ofthe inclined portion of the trench-periphery portion 132B.

In the case of forming the first insulating film 131 before the secondinsulating film 132, also, wet etching may be performed after formationof the second insulating film 132. By performing wet etching, it isensured to expose the first insulating film 131 on the side of thetrench 102 a, to bring the first insulating film 131 into contact withthe gate electrode 105 on the side of the trench 102 a. FIG. 18 is across-sectional view showing a sixth alteration of the semiconductordevice of the embodiment. When the second insulating film 132 iswet-etched, as shown in FIG. 18, the end of the trench-periphery portion132B recedes from the top edge of the trench 102 a after formation ofthe first insulating film 131 by t4. The receding amount t4 of thetrench-periphery portion 132B approximately corresponds with the etchingamount of the second insulating film 132. Thirty percent or less of thethickness of the second insulating film 132 will be enough as theetching amount of the second insulating film 132. In addition, in thiscase, since the first insulating film 131 has been formed also on theperiphery of the trench 102 a, there will be little influence on thegate capacitance as far as the receding amount t4 is about 100 nm orless.

Moreover, as shown in FIG. 19, wet etching of the second insulating film132 may be performed even when the top end portion of the trench 102 ais rounded. FIG. 19 is a cross-sectional view showing a seventhalteration of the semiconductor device of the embodiment. In this case,the inclination θ5 of a portion of the trench 102 a having the firstinsulating film 131 formed thereon that is in contact with the end ofthe trench-periphery portion 132B is smaller than the inclination θ1 ofthe inclined portion of the trench-periphery portion 132B.

The thickness of the first insulating film 131 changes with the planedirection of the semiconductor layer 102. When the semiconductor layer102 is formed on the substrate 101 having the (0001) Si plane as theprincipal surface, the thickness of the portions of the first insulatingfilm 131 formed on the top surface of the semiconductor layer 102 andthe bottom of the trench 102 a is smaller than the thickness of theportion thereof formed on the side of the trench 102 a. However, thethin first insulating film 131 will cause no problem because the secondinsulating film 132 is to be formed on the bottom of the trench 102 aand the periphery thereof.

While the n-type MISFET was described in this embodiment, a p-typeMISFET can also be formed. In this case, the conductivity type of thesubstrate 101, the drift region 121, and the source region 124 may bechanged to the p-type and that of the body region 123 to the n-type.Also, the semiconductor layer 102 may have a region other than the driftregion 121, the body region 123, and the source region 124. For example,for reducing the electric field, an impurity layer having a conductivitytype different from the drift region 121 may be provided near the bottomof the trench 102 a.

While the MISFET having an inverted channel structure was described inthis embodiment, a similar configuration can also be used for a MISFEThaving a stored channel structure as shown in FIG. 20. FIG. 20 is across-sectional view showing an eighth alteration of the semiconductordevice of the embodiment. For example, after formation of the trench 102a in the semiconductor layer 102, a channel layer 125 made of an n-typeSiC layer may be formed on the semiconductor layer 102 including theinside of the trench 102 a. After formation of the channel layer 125,the gate insulating film 103, the gate electrode 105, the sourceelectrode 106, the drain electrode 107, etc. may be formed as in thecase of the MISFET having the inverted channel structure. While theexample of forming the first insulating film 131 after the secondinsulating film 132 was shown in FIG. 20, the first insulating film 131may be formed before the second insulating film 132. In the case of thestored channel structure, also, wet etching may be performed afterformation of the second insulating film 132, and a top portion of thetrench 102 a may be rounded. Also, a p-type MISFET may be formed.

Furthermore, not only the MISFETs, but various types of semiconductordevices that have an electrode placed on a semiconductor layer via aninsulating film can be fabricated in a similar manner. For example, aninsulated gate bipolar transistor (IGBT) can be formed by impartingdifferent conductivity types to the substrate and the semiconductorlayer directly formed thereon.

While the example of a plurality of unit cells arranged in a zigzagfashion was described in this embodiment, the unit cells may be arrangedin any fashion. Also, while the example of the trench having a squareplanar shape was described, the trench may have any planar shape. Forexample, the trench may have a rectangular planar shape, and the unitcells may be arranged so that the major sides of a plurality of trenchesextend in parallel with each other.

In this embodiment, the example of the substrate 101 being made of4H—SiC and the semiconductor layer 102 being formed on the (0001) Siplane was described. Alternatively, the semiconductor layer 102 may beformed on a (000-1) C plane and the drain electrode 107 may be formed onthe (0001) Si plane. Otherwise, the plane direction of the principalsurface may be of another crystal plane. Further, another polytype SiCsubstrate may be used.

While the semiconductor device using SiC was described in thisembodiment, the present disclosure is also applicable to a semiconductordevice using another wide band-gap semiconductor such as gallium nitride(GaN) and diamond. Also, the present disclosure is applicable to asemiconductor device using silicon.

INDUSTRIAL APPLICABILITY

The semiconductor device and the fabrication method for the same of thepresent disclosure are useful as various types of semiconductor devicesincluding power devices and fabrication methods for the same.

DESCRIPTION OF REFERENCE CHARACTERS

-   11 Unit Cell-   101 Substrate-   102 Semiconductor Layer-   102 a Trench-   103 Gate Insulating Film-   105 Gate Electrode-   105A Conductive Film-   106 Source Electrode-   107 Drain Electrode-   121 Drift Region-   123 Body Region-   124 Source Region-   125 Channel Layer-   131 First Insulating Film-   132 Second Insulating Film-   132A Inner-Trench Portion-   132B Trench-Periphery Portion-   141 Resist Layer

1. A fabrication method for a semiconductor device, comprising the stepsof: preparing a substrate having a semiconductor layer provided on aprincipal surface; forming a trench in the semiconductor layer; forminga gate insulating film on a side of the trench, a bottom of the trench,and a periphery of the trench; and forming a conductive film on the gateinsulating film to fill the trench and extend on the periphery of thetrench, wherein the step of forming a gate insulating film includes astep of forming a first insulating film on the side of the trench and astep of forming a second insulating film on the bottom of the trench andthe periphery of the trench using a high-density plasma chemical vapordeposition method, the thickness of portions of the gate insulating filmformed on the bottom of the trench and the periphery of the trench beingmade larger than that of a portion of the gate insulating film formed onthe side of the trench, and in the step of forming a conductive film,the conductive film is formed to be in contact with a portion of thefirst insulating film formed on the side of the trench.
 2. Thefabrication method for a semiconductor device of claim 1, wherein thestep of forming a first insulating film is performed after the step offorming a second insulating film.
 3. The fabrication method for asemiconductor device of claim 1, wherein the step of forming a firstinsulating film is performed before the step of forming a secondinsulating film.
 4. The fabrication method for a semiconductor device ofclaim 1, wherein the step of forming a trench includes a step ofrounding a top end portion of the trench.
 5. The fabrication method fora semiconductor device of claim 1, further comprising the step of:wet-etching the second insulating film before the step of forming aconductive film, wherein the etching amount of the second insulatingfilm is 30% or less of the thickness of the second insulating film. 6.The fabrication method for a semiconductor device of claim 1, wherein inthe step of preparing a substrate, the semiconductor layer is formed tohave a drift region of a first conductivity type and a body region of asecond conductivity type provided on the drift region, in the step offorming a trench, the trench is formed so that the bottom thereof islocated below an interface between the drift region and the body regionand above the bottom of the drift region, and in the step of forming agate insulating film, the gate insulating film is formed so that the topsurface of the portion thereof formed on the bottom of the trench islocated below the interface between the drift region and the bodyregion.
 7. The fabrication method for a semiconductor device of claim 1,wherein the semiconductor layer is made of a wide band-gapsemiconductor.
 8. The fabrication method for a semiconductor device ofclaim 7, wherein the wide band-gap semiconductor is silicon carbide. 9.The fabrication method for a semiconductor device of claim 8, whereinthe substrate is a silicon carbide substrate, and the principal surfaceis a silicon plane.
 10. A semiconductor device comprising: a substrate;a semiconductor layer provided on a principal surface of the substrate;a trench provided in the semiconductor layer; a gate insulating filmprovided on a side of the trench, a bottom of the trench, and aperiphery of the trench; and a conductive film provided on the gateinsulating film to fill the trench and extend on the periphery of thetrench, wherein the gate insulating film has a first insulating filmprovided on the side of the trench and a second insulating film providedon the bottom of the trench and the periphery of the trench, thethickness of portions of the gate insulating film provided on the bottomof the trench and the periphery of the trench is larger than that of aportion of the gate insulating film provided on the side of the trench,a portion of the second insulating film provided on the periphery of thetrench has an inclined portion that becomes gradually thicker from thetrench-side end, and the inclination angle of the inclined portion withrespect to the principal surface of the substrate is 45±5 degrees, andthe conductive film is in contact with a portion of the first insulatingfilm formed on the side of the trench.
 11. The semiconductor device ofclaim 10, wherein the first insulating film is formed on the side of thetrench and the bottom and periphery of the trench.
 12. The semiconductordevice of claim 11, wherein the position of the trench-side end of theportion of the second insulating film provided on the periphery of thetrench corresponds with the position of a portion of the firstinsulating film corresponding to a top edge of the trench.
 13. Thesemiconductor device of claim 11, wherein the spacing between thetrench-side end of the portion of the second insulating film provided onthe periphery of the trench and a portion of the first insulating filmcorresponding to a top edge of the trench is 30% or less of thethickness of the portion of the second insulating film provided on theperiphery of the trench.
 14. The semiconductor device of claim 11,wherein a top end portion of the trench is rounded, and the angle of atangent at a portion of the top end portion the trench that is incontact with the trench-side end of the second insulating film withrespect to the principal surface of the substrate is the same as theinclination angle of the inclined portion.
 15. The semiconductor deviceof claim 11, wherein a top end portion of the trench is rounded, and theangle of a tangent at a portion of the top end portion the trench thatis in contact with the trench-side end of the second insulating filmwith respect to the principal surface of the substrate is smaller thanthe inclination angle of the inclined portion.
 16. The semiconductordevice of claim 10, wherein the semiconductor layer has a drift regionof a first conductivity type and a body region of a second conductivitytype provided on the drift region, the bottom of the trench is locatedbelow an interface between the drift region and the body region andabove the bottom of the drift region, and the top surface of the portionof the gate insulating film formed on the bottom of the trench islocated below the interface between the drift region and the bodyregion.
 17. The semiconductor device of claim 10, wherein thesemiconductor layer is made of a wide band-gap semiconductor.
 18. Thesemiconductor device of claim 17, wherein the wide band-gapsemiconductor is silicon carbide.
 19. The semiconductor device of claim18, wherein the substrate is a silicon carbide substrate, and theprincipal surface is a silicon surface.
 20. The semiconductor device ofclaim 10, wherein the thickness of the portion of the gate insulatingfilm provided on the periphery of the trench is larger than that of theportion of the gate insulating film provided on the bottom of thetrench.